In SDN, two functional modules of a traditional network device that are configured for data forwarding and management control are separated, and a centralized control device is used to configure, manage, and control various network devices using a standardized interface. A switching device switch strictly performs forwarding based on a flow table, the switch provides an interface for the control device for control, and a specific protocol is followed between the switch and the controller.
In the SDN, features of the switch include the following. 1. The flow table is initially empty. 2. When a routing table that match a received data packet is not found in the switch, reporting to the Controller is performed to request a forwarding policy. A feature of the controller includes possessing a global view of the entire network, and formulating a delivery policy for a request reported by the switch.
There are generally two networking modes for the SDN network, out-of-band connection control (out-of-band mode) and in-band connection control (in-band mode).
(1) Out-of-band connection mode: The control device is directly connected to each switching device, and there is an independent control channel in the network. Control information and data information are transmitted without affecting each other, and control signaling has relatively high security. However, in this mode, each switch in the network needs to have a designated port for forwarding control signaling, which consumes more port resources.
(2) In-band connection mode: The control device needs to be connected to only some switching devices, and implements communication with another switching device using a data plane link. In this mode, transmission of control information and transmission of data information are mutually affected, and security is relatively low.
In the in-band connection mode, the control information and the data information share a same transmission channel, and therefore, forwarding of the data information may cause congestion of the transmission channel, and affect effective transmission of the control information. The control device loses a capability of controlling multiple switching devices if the channel for transmitting the control information fails due to congestion.